Semiconductor device structure with semiconductor wire

ABSTRACT

Semiconductor device structures are provided. The semiconductor device structure includes first semiconductor wires over a semiconductor substrate. The first semiconductor wires are vertically spaced apart from each other. The semiconductor device structure also includes a gate stack surrounding first portions of the first semiconductor wires, and a spacer element surrounding second portions of the first semiconductor wires. The first portions have a first width and the second portions have a second width. In addition, the semiconductor device structure includes a second semiconductor wire between the second portions. The second semiconductor wire has a third width, and the third width is substantially equal to the second width and greater than the first width.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/692,188, filed Aug. 31, 2017 and entitled “Semiconductor devicestructure with semiconductor wire”, now U.S. Pat. No. 10,290,548, theentirety of which is incorporated by reference herein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs. Each generation has smaller and more complexcircuits than the previous generation.

In the course of IC evolution, functional density (i.e., the number ofinterconnected devices per chip area) has generally increased whilegeometric size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling-downprocess generally provides benefits by increasing production efficiencyand lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,a three-dimensional transistor, such as a semiconductor device withnanowires, has been introduced to replace a planar transistor. Theserelatively new types of semiconductor IC devices face manufacturingchallenges, and they have not been entirely satisfactory in allrespects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A-1L are perspective views of various stages of a process forforming a semiconductor device structure, in accordance with someembodiments.

FIG. 1I-1 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line I-I′ shown in FIG. 1I, inaccordance with some embodiments.

FIG. 1I-2 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line II-II′ shown in FIG. 1I, inaccordance with some embodiments.

FIG. 1I-3 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line shown in FIG. 1I, in accordancewith some embodiments.

FIG. 1L-1 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line I-I′ shown in FIG. 1L, inaccordance with some embodiments.

FIG. 1L-2 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line II-II′ shown in FIG. 1L, inaccordance with some embodiments.

FIG. 1L-3 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line III-III′ shown in FIG. 1L, inaccordance with some embodiments.

FIGS. 2A-2B are perspective views of various stages of a process forforming a semiconductor device structure, in accordance with someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matterprovided. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and they are not intended to be limiting. For example, theformation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Furthermore, spatially relative terms, such as “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Embodiments of the disclosure form a semiconductor device structure withgate all around (GAA) transistor structures. The GAA transistorstructures may be patterned by any suitable method. For example, thestructures may be patterned using one or more photolithographyprocesses, including double-patterning or multi-patterning processes.Generally, double-patterning or multi-patterning processes combinephotolithography and self-aligned processes, allowing patterns to becreated that have, for example, pitches smaller than what is otherwiseobtainable using a single, direct photolithography process. For example,in one embodiment, a sacrificial layer is formed over a substrate andpatterned using a photolithography process. Spacers are formed alongsidethe patterned sacrificial layer using a self-aligned process. Thesacrificial layer is then removed, and the remaining spacers may then beused to pattern the GAA structure.

Some embodiments of the disclosure are described. Additional operationscan be provided before, during, and/or after the stages described inthese embodiments. Some of the stages that are described can be replacedor eliminated for different embodiments. Additional features can beadded to the semiconductor device structure. Some of the featuresdescribed below can be replaced or eliminated and additional featurescan be added for different embodiments. Although some embodiments arediscussed with operations performed in a particular order, theseoperations may be performed in another logical order.

FIGS. 1A-1L are perspective views of various stages of a process forforming a semiconductor device structure, in accordance with someembodiments. As shown in FIG. 1A, a semiconductor substrate 100 (such asa wafer) is provided. In some embodiments, the semiconductor substrate100 is a bulk semiconductor substrate. The semiconductor substrate 100may include silicon or another elementary semiconductor material (suchas germanium) in a single crystal, polycrystal, or amorphous structure.In some other embodiments, the semiconductor substrate 100 includes acompound semiconductor. The compound semiconductor may include germaniumtin, silicon germanium tin, gallium arsenide, silicon carbide, anothersuitable compound semiconductor, or a combination thereof. In someembodiments, the semiconductor substrate 100 includes asemiconductor-on-insulator (SOI) substrate (such as silicon on insulatoror germanium on insulator).

As shown in FIG. 1A, multiple semiconductor layers 110 and 120 arealternately deposited over the semiconductor substrate 100, inaccordance with some embodiments. Therefore, the semiconductor layers110 and 120 are vertically stacked and positioned at different levels.Although FIG. 1A shows three semiconductor layers 110 and threesemiconductor layers 120, embodiments of the disclosure are not limitedthereto. There may be more or less semiconductor layers 110 and 120 thanthose shown in FIG. 1A.

The semiconductor layers 110 and 120 have different thicknesses. In someembodiments, each of the semiconductor layers 110 is thicker than eachof the semiconductor layers 120. In some embodiments, the thickness T₁of each of the semiconductor layers 110 is in a range from about 7 nm toabout 8 or 9 nm. In some embodiments, the thickness T₂ of each of thesemiconductor layers 120 is in a range from about 4 nm to about 5 or 6nm. In some embodiments, the difference between the thickness T₁ and T₂is in a range from about 1 nm to about 5 nm. In some embodiments, aratio of the thickness T₁ to the thickness T₂ is in a range from about1.16 to about 2.25. It should be noted that these ranges are onlyexamples and are not a limitation to the disclosure. In some otherembodiments, each of the semiconductor layers 110 is thinner than eachof the semiconductor layers 120.

In some embodiments, the semiconductor layers 110 and 120 includesilicon, silicon germanium, germanium tin, silicon germanium tin, oranother suitable semiconductor material. In some embodiments, thesemiconductor layers 110 are made of a different material than that ofthe semiconductor layers 120. In some embodiments, the semiconductorlayers 110 are made of silicon germanium, and the semiconductor layers120 are made of silicon. In these embodiments, the thickness T₁ isgreater than the thickness T₂. In some other embodiments, thesemiconductor layers 110 are made of silicon, and the semiconductorlayers 120 are made of silicon germanium. In these embodiments, thethickness T₁ is less than the thickness T₂.

In some embodiments, the semiconductor layers 110 and 120 are formedusing an epitaxial growth process. Each of the semiconductor layers 110and 120 may be formed using a selective epitaxial growth (SEG) process,a chemical vapor deposition (CVD) process (e.g., a vapor-phase epitaxy(VPE) process, a low pressure CVD process, and/or an ultra-high vacuumCVD process), a molecular beam epitaxy process, other applicableprocesses, or a combination thereof. In some embodiments, thesemiconductor layers 110 and 120 are grown in-situ in the same processchamber.

As shown in FIG. 1B, multiple recesses (or trenches) 130 are formed topattern the semiconductor layers 110 and 120, in accordance with someembodiments. As a result, multiple fin structures are formed between therecesses 130. One fin structure 140 is shown in FIG. 1B as an example.The semiconductor layers 110 and 120 are patterned into semiconductorwires 110′ and 120′, respectively. The stacked semiconductor wires 110′and 120′ form the fin structure 140. The semiconductor wires 110′ and120′ may be also referred to as nanowires. In some embodiments, thewidth W₁ of the semiconductor wires 110′ and 120′ is in a range fromabout 7 nm to about 8 or 9 nm.

In some embodiments, the recesses 130 further extend into thesemiconductor substrate 100. As a result, the fin structure 140 furtherincludes a fin portion of the semiconductor substrate 100 between therecesses 130, as shown in FIG. 1B. In some embodiments, multiplephotolithography processes and etching processes are performed to formthe recesses 130. Embodiments of the disclosure are not limited thereto.In some other embodiments, the recesses 130 do not extend into thesemiconductor substrate 100 and the semiconductor substrate 100 does nothave a fin portion.

As shown in FIG. 1C, isolation features 150 are formed in the recesses130, in accordance with some embodiments. The isolation features 150 areused to define and electrically isolate various device elements formedin and/or over the semiconductor substrate 100. For example, theisolation features 150 may be used to separate two neighboring fieldeffect transistors (FETs). In some embodiments, the isolation features150 include shallow trench isolation (STI) features, local oxidation ofsilicon (LOCOS) features, other suitable isolation features, or acombination thereof.

In some embodiments, the isolation features 150 are made of a dielectricmaterial. The dielectric material may include silicon oxide, siliconnitride, silicon oxynitride, spin-on glass, low-K dielectric material,other suitable materials, or a combination thereof. In some embodiments,a dielectric material layer is deposited over the semiconductorsubstrate 100. The dielectric material layer covers the fin structure140 and fills the recesses 130. In some embodiments, the dielectricmaterial layer is deposited using a CVD process, a spin-on process,another applicable process, or a combination thereof. In someembodiments, a planarization process is subsequently performed to thindown the dielectric material layer until the fin structure 140 (such asthe topmost of the semiconductor wires 120′) are exposed. Theplanarization process may include a chemical mechanical polishing (CMP)process, a grinding process, an etching process, another applicableprocess, or a combination thereof. Afterwards, the dielectric materiallayer is etched back to form the isolation features 150. In someembodiments, the semiconductor wires 110′ and 120′ are not surrounded orcovered by the isolation features 150.

Afterwards, multiple dummy or sacrificial gate stacks are formed overthe semiconductor substrate 100, in accordance with some embodiments.One dummy gate stack 160 is shown in FIG. 1D as an example. The dummygate stack 160 covers portions of the fin structure 140 and theisolation features 150.

In some embodiments, the dummy gate stack 160 includes a dummydielectric layer 170, a dummy gate electrode 180, and a hard mask 190.The dummy dielectric layer 170 is deposited over the top and thesidewalls of the fin structure 140, in accordance with some embodiments.In some embodiments, the dummy dielectric layer 170 is made of adielectric material, such as silicon oxide. In some embodiments, thedummy dielectric layer 170 is conformally deposited over the finstructure 140. The dummy dielectric layer 170 may be deposited using aCVD process, an atomic layer deposition (ALD) process, a physical vapordeposition (PVD) process, a spin-on process, another applicable process,or a combination thereof.

In some embodiments, the dummy gate electrode 180 is made ofpolysilicon. The dummy gate electrode 180 will be replaced with anotherconductive material (such as a metal material). The hard mask 190 isused to assist in the patterning process for forming the dummy gatestack 160. In some embodiments, the hard mask 190 is made of siliconoxide, silicon nitride, silicon oxynitride, silicon carbide, anothersuitable material, or a combination thereof. In some embodiments, thehard mask 190 has a multi-layer structure. Many variations andmodifications can be made to embodiments of the disclosure. In someother embodiments, the hard mask 190 is not formed.

In some embodiments, a dummy gate electrode layer and one or more hardmask layers are deposited over the dummy dielectric layer 170.Afterwards, a photolithography process and an etching process areperformed to pattern the hard mask layer so as to form the hard mask190. With the assistance of the hard mask 190, the dummy gate electrodelayer is patterned. As a result, the dummy gate electrode 180 is formed.During the etching process for forming the dummy gate electrode 180, thedummy dielectric layer 170 may serve as an etch stop layer to protectthe fin structure 140 under the dummy dielectric layer 170. Afterwards,the portions of the dummy dielectric layer 170 that are not covered bythe dummy gate electrode 180 are removed. As a result, the dummydielectric layer 170 is patterned, and the dummy gate stack 160 isformed, as shown in FIG. 1D.

As shown in FIG. 1D, spacer elements 200 are formed over the sidewallsof the dummy gate stack 160, in accordance with some embodiments. Thespacer elements 200 may be used to assist in the formation of source ordrain structures (or regions) in subsequent processes. In someembodiments, the spacer elements 200 partially cover the top portionsand the sidewalls of the fin structure 140.

In some embodiments, the spacer elements 200 are made of a dielectricmaterial. The dielectric material may include silicon carbon nitride,silicon nitride, silicon oxynitride, silicon carbide, another suitabledielectric material, or a combination thereof. In some embodiments, aspacer layer is conformally deposited over the semiconductor substrate100. The spacer layer may be deposited using a CVD process, a PVDprocess, a spin-on process, another applicable process, or a combinationthereof. Afterwards, an etching process, such as an anisotropic etchingprocess, is performed to partially remove the spacer layer. As a result,the remaining portions of the spacer layer over the sidewalls of thedummy gate stack 160 form the spacer elements 200.

As shown in FIG. 1E, the portions of the semiconductor wires 110′ thatare not under the dummy gate stack 160 and the spacer elements 200 areremoved, in accordance with some embodiments. In some embodiments, anetching process is used to partially remove the semiconductor wires110′. As a result, space is created for a subsequent formation processof source or drain structures. The portions 120C of the semiconductorwires 120′ that are not under the dummy gate stack 160 and the spacerelements 200 are not removed.

As shown in FIG. 1F, source or drain structures 210 are formed over thesemiconductor substrate 100, in accordance with some embodiments. Thesource or drain structures 210 surround the portions 120C of thesemiconductor wires 120′. The source or drain structures 210 may be usedto provide stress or strain to channel regions. As a result, the carriermobility of the device and device performance are improved.

In some embodiments, a semiconductor material (or two or moresemiconductor materials) is epitaxially grown over the semiconductorsubstrate 100 so as to form the source or drain structures 210. In someembodiments, the source or drain structures 210 are a P-typesemiconductor material. In some embodiments, the source or drainstructures 210 are an N-type semiconductor material. The source or drainstructures 210 may include epitaxially grown silicon, silicon-germanium,epitaxially grown phosphorous-doped silicon (Si:P), boron-doped silicongermanium (SiGe:B), or another suitable semiconductor material. In someembodiments, the source or drain structures 210 are formed using a SEGprocess, a CVD process (e.g., a VPE process, a LPCVD process, and/or anUHV-CVD process), a molecular beam epitaxy process, deposition of dopedamorphous semiconductor followed by a solid-phase epitaxialrecrystallization step, another applicable process, or a combinationthereof.

In some embodiments, the source or drain structures 210 are doped withone or more suitable dopants. For example, the source or drainstructures 210 are Si source or drain features doped with phosphorus,arsenic, antimony, or another suitable dopant. Alternatively, the sourceor drain structures 210 are SiGe source or drain features doped withboron or another suitable dopant. In some embodiments, multipleimplantation processes are performed to dope the source or drainstructures 210. In some embodiments, the source or drain structures 210are doped in-situ during the growth of the source or drain structures210. In some other embodiments, the source or drain structures 210 arenot doped during the growth of the source or drain structures 210. Afterthe epitaxial growth, the source or drain structures 210 are doped in asubsequent process. In some embodiments, the doping is achieved using anion implantation process, a plasma immersion ion implantation process, agas and/or solid source diffusion process, another applicable process,or a combination thereof. In some embodiments, the source or drainstructures 210 are further exposed to an annealing process (such as arapid thermal annealing process) to activate the dopants.

As shown in FIG. 1F, a dielectric layer 220 is deposited over thesemiconductor substrate 100, in accordance with some embodiments. Thedielectric layer 220 serves as an interlayer dielectric (ILD) layer. Insome embodiments, the dielectric layer 220 is made of silicon oxide,silicon oxynitride, borosilicate glass (BSG), phosphosilicate glass(PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass(FSG), low-K material, porous dielectric material, another suitabledielectric material, or a combination thereof.

In some embodiments, the dielectric layer 220 is deposited using a CVDprocess, a spin-on process, an ALD process, a PVD process, anotherapplicable process, or a combination thereof. In some embodiments, thedielectric layer 220 covers the isolation features 150, the dummy gatestack 160, the spacer elements 200, and the source or drain structures210. Afterwards, the dielectric layer 220 is thinned down until thedummy gate stack 160 and the spacer elements 200 are exposed. In someembodiments, a planarization process (such as a CMP process) isperformed to thin down the dielectric layer 220.

In accordance with some embodiments, the semiconductor device structureis divided into multiple regions 100A and 100B. In some embodiments, anN-type FET is configured to form in the region 100A while a P-type FETis configured to form in the region 100B. Afterwards, a first mask layer(not shown) is formed over the dielectric layer 220 in the regions 100Aand 100B to assist in the subsequent etching processes. In someembodiments, the first mask layer covers the regions 100A and 100Bwithout covering the dummy gate stack 160 in the region 100A. In someembodiments, the first mask layer is a patterned photoresist layer.

As shown in FIG. 1G, the dummy gate stack 160 in the region 100A isremoved, in accordance with some embodiments. As a result, a recess 230is formed over the semiconductor substrate 100 in the region 100A. Insome embodiments, the dummy gate stack 160 is removed using a wetetching process, a dry etching process, another applicable process, or acombination thereof.

In some embodiments, the portions 110A of the semiconductor wires 110′are exposed from the recess 230, and the portions 110B of thesemiconductor wires 110′ are covered by the spacer elements 200, asshown in FIG. 1G. The portions 120A of the semiconductor wires 120′ areexposed from the recess 230. The portions 120B of the semiconductorwires 120′ are covered by the spacer elements 200. The portions 120C ofthe semiconductor wires 120′ are covered by the source or drainstructures 210. The portions 110B, 120B and 120C are illustrated asdashed lines so that they are still visible in FIG. 1G for a betterunderstanding of the structure.

As shown in FIG. 1H, the portions 110A of the semiconductor wires 110′are removed, in accordance with some embodiments. As a result, theportions 120A of the semiconductor wires 120′ suspend between the spacerelements 200. The portions 120A serve as channel regions of FETs in theregion 100A.

In some embodiments, the portions 120A have a rectangularcross-sectional profile since the width W₁ of the portions 120A isgreater than the thickness T₂ of the portions 120A. The corners and/oredges of the portions 120A may be relatively rounded. In some otherembodiments, the portions 120A have an oval cross-sectional profile. Insome embodiments, the cross-sectional profile of the portions 120A isnot square or circular.

The portions 110B of the semiconductor wires 110′ remain covered by thespacer elements 200, as shown in FIG. 1H. The portions 120A are spacedapart or separated from one another by an interval (or spacing) that issubstantially equal to the thickness T₁ of the portions 110B. Theinterval between the portions 120A is different from the thickness T₂ ofthe portions 120A. In some embodiments, the interval between theportions 120A is greater than the thickness T₂. Therefore, it becomesmuch easier to enclose the portions 120A with a subsequently formedmetal gate stack. The process window of the metal gate stack isincreased without enlarging the total thickness of the semiconductorwires 110′ and 120′ (i.e., the height of the fin structure 140 shown inFIG. 1B). In addition, the driving current of the semiconductor devicestructure is also prevented from degradation, which may be induced byreducing the number of semiconductor wires for increasing the intervalor space between semiconductor wires.

In some embodiments, an etching process is performed to selectivelyremove the portions 110A of the semiconductor wires 110′ withoutremoving the portions 120A of the semiconductor wires 120′. In someembodiments, the etching process includes a wet etching process, a dryetching process, or another suitable etching process. In someembodiments, the etchant used in the etching process includes a liquidmixture. The liquid mixture may include NH₄OH, ammonia-peroxide mixture(APM), tetramethyl ammonium hydroxide (TMAH), another suitable solution,or a combination thereof. In some other embodiments, the etchant used inthe etching process includes a gas mixture. The gas mixture may includeCF₄, SF₆, CH₃F, another suitable gas, or a combination thereof. Duringthe etching operations, the composition of the etchant may be variedaccording to requirements. In some embodiments, the etchant has asufficiently high etch selectivity of the semiconductor wires 110′ tothe semiconductor wires 120′. As a result, the semiconductor wires 110′are etched much faster than the semiconductor wires 120′.

As shown in FIG. 1I, a metal gate stack 240 is formed in the recess 230in the region 100A, in accordance with some embodiments. The metal gatestack 240 encircles the portions 120A of the semiconductor wires 120′.For illustration purposes, the portions 120A covered by the metal gatestack 240 are not shown in FIG. 1I.

As shown in FIG. 1I, the metal gate stack 240 includes an interfaciallayer 250, a gate dielectric layer 260, and metal gate stacking layers270, 280 and 290, in accordance with some embodiments. The interfaciallayer 250 is used to reduce stress between the gate dielectric layer 260and the portions 120A of the semiconductor wires 120′. The interfaciallayer 250 may include silicon oxide or another suitable material. Insome embodiments, the interfacial layer 250 is formed using an ALDprocess, a thermal oxidation process, another applicable process, or acombination thereof.

In some embodiments, the gate dielectric layer 260 includes a high-kmaterial layer. The term “high-k material” may mean a material having adielectric constant greater than the dielectric constant of silicondioxide. The high-K dielectric layer may be made of hafnium oxide,zirconium oxide, aluminum oxide, silicon oxynitride, hafnium siliconoxynitride, hafnium tantalum oxide, another suitable high-K material, ora combination thereof. In some embodiments, the gate dielectric layer260 is deposited using an ALD process, a CVD process, a spin-on process,another applicable process, or a combination thereof. In someembodiments, a high-temperature annealing operation is performed toreduce or eliminate defects in the gate dielectric layer 260.

In some embodiments, the metal gate stacking layers 270, 280 and 290include a barrier layer, one or more work function layers, a blockinglayer, a glue layer, a metal filling layer, one or more other suitablemetal gate layers, and combinations thereof. Although FIG. 1I showsthree metal gate stacking layers 270, 280 and 290, embodiments of thedisclosure are not limited thereto. There may be more or less metal gatestacking layers than those shown in FIG. 1I.

More specifically, the barrier layer may be used to interface the gatedielectric layer 260 with the work function layer. The barrier layer mayalso be used to prevent diffusion between the gate dielectric layer 260and the work function layer. The blocking layer may be used to preventthe metal filling layer from diffusing or penetrating into the workfunction layer. The glue layer may be used to increase the adhesionbetween the work function layer and the metal filling layer so as toprevent peeling or delamination of the metal filling layer. In someembodiments, each of the barrier layer, the blocking layer and the gluelayer is made of or includes tantalum nitride, titanium nitride, anothersuitable material, or a combination thereof. In some embodiments, themetal filling layer is made of or includes tungsten, aluminum, copper,cobalt, another suitable material, or a combination thereof.

The work function layer is used to provide the desired work function fortransistors to enhance device performance including improved thresholdvoltage (Vt). In the embodiments of forming an NMOS transistor, the workfunction layer can be an N-type metal layer, which is capable ofproviding a work function value suitable for the device. The workfunction value may be substantially equal to or less than about 4.5 eV.The N-type metal layer may include metal, metal carbide, metal nitride,or a combination thereof. For example, the N-type metal layer includestitanium nitride, tantalum, tantalum nitride, another suitable material,or a combination thereof.

On the other hand, in the embodiments of forming a PMOS transistor, thework function layer can be a P-type metal layer, which is capable ofproviding a work function value suitable for the device. The workfunction value may be substantially equal to or greater than about 4.8eV. The P-type metal layer may include metal, metal carbide, metalnitride, other suitable materials, or a combination thereof. Forexample, the P-type metal includes tantalum nitride, tungsten nitride,titanium, titanium nitride, another suitable material, or a combinationthereof.

The thickness and/or the composition of the work function layer may befine-tuned to adjust the work function level. For example, a titaniumnitride layer may be used as a P-type metal layer or an N-type metallayer, depending on the thickness and/or the composition of the titaniumnitride layer.

The metal gate stacking layers 270, 280 and 290 are sequentiallydeposited by using suitable deposition methods. The suitable depositionmethods may include an ALD process, a PVD process, an electroplatingprocess, an electroless plating process, a CVD process, other applicableprocesses, or a combination thereof. In some embodiments, theinterfacial layer 250, the gate dielectric layer 260, and the metal gatestacking layers 270, 280 and 290 fill the recess 230. In someembodiments, portions of the interfacial layer 250, the gate dielectriclayer 260, and the metal gate stacking layers 270, 280 and 290 outsideof the recess 230 are removed using a planarization process (such as aCMP process). The planarization process is performed until thedielectric layer 220 is exposed. As a result, portions of theinterfacial layer 250, the gate dielectric layer 260, and the metal gatestacking layers 270, 280 and 290 remaining in the recess 230 form themetal gate stack 240, as shown in FIG. 1I.

FIG. 1I-1 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line I-I′ shown in FIG. 1I, inaccordance with some embodiments. The sectional line I-I′ extends alongthe semiconductor wires 120′. FIG. 1I-2 is a cross-sectional view of thesemiconductor device structure taken along a sectional line II-IF shownin FIG. 1I, in accordance with some embodiments. The sectional lineII-IF extends along the metal gate stack 240. FIG. 1I-3 is across-sectional view of the semiconductor device structure taken along asectional line shown in FIG. 1I, in accordance with some embodiments.The sectional line extends along the spacer elements 200.

As shown in FIG. 1I-1, the portions 120A of the semiconductor wires 120′are surrounded by the metal gate stack 240 in the region 100A, inaccordance with some embodiments. The portions 120B of the semiconductorwires 120′ are vertically sandwiched between the portions 110B of thesemiconductor wires 110′. The portions 120C of the semiconductor wires120′ are surrounded by the source or drain structures 210. The portions120C may have substantially the same cross-sectional profile and area(such as the width and thickness) as the portions 120A and/or theportions 120B.

As shown in FIG. 1I-2, the portions 120A of the semiconductor wires 120′are embedded in the metal gate stack 240 in the region 100A, inaccordance with some embodiments. In some embodiments, the interfaciallayer 250, the gate dielectric layer 260, and all the metal gatestacking layers 270, 280 and 290 are between two of the portions 120A.In some other embodiments, the interfacial layer 250, the gatedielectric layer 260, and more than one of the metal gate stackinglayers 270, 280 and 290 are between two of the portions 120A.

Since the space between the portions 120A is large enough, the metalgate stack 240, especially the work function layer(s), can be uniformlyformed between the portions 120A. As shown in FIGS. 1I-1 and 1I-2, themetal gate stacking layers 270, 280 and 290 uniformly surround each ofthe portions 120A. The uniform work function level results in uniformthreshold voltage. Therefore, the device performance of thesemiconductor device structure at on-state is improved and currentleakage at off-state is avoided.

In accordance with some embodiments, the width W₁ of the portions 120Ais greater than the thickness T₂ of the portions 120A, as shown in FIG.1I-2. In some embodiments, a ratio of the thickness T₂ to the width W₁is in a range from about 0.3 to about 0.6. The mobility of electrons inthe portions 120A (such as silicon) is large or the maximum in the (100)crystalline plane and in the [100] crystalline direction, as indicatedby the dashed arrow in FIG. 1I-2. The width W₁ is along the [100]crystalline direction. As a result, the effective area of channelregions or effective channel width (W_(eff)) of the transistor in theregion 100A is greatly increased. The operation speed of the transistorin the region 100A can be improved. Accordingly, the device performanceof the semiconductor device structure at on-state is significantlyenhanced.

As shown in FIG. 1I-3, the vertically stacked portions 110B and 120B arecovered by the spacer elements 200 in the region 100A, in accordancewith some embodiments. In some embodiments, the width W₁ issubstantially equal to the thickness T₁ of the portions 110B.Accordingly, the cross-sectional profile of the portions 110B may besquare. However, embodiments of the disclosure are not limited thereto.The width W₁ may be greater or less than the thickness T₁. In someembodiments, the width W₁ is greater than the thickness T₂ of theportions 120B. The width W₁ may be slightly less than the thickness T₁but greater than the thickness T₂. The portions 120B shown in FIG. 1I-3and the portions 120A shown in FIG. 1I-2 may have slightly differentprofile. The portions 120B shown in FIG. 1I-3 may have a slightly largerarea than the portions 120A shown in FIG. 1I-2.

Afterwards, a second mask layer (not shown) is formed over thedielectric layer 220 in the regions 100A and 100B to assist in thesubsequent etching processes. In some embodiments, the second mask layercovers the regions 100A and 100B without covering the dummy gate stack160 in the region 100B. In some embodiments, the second mask layer is apatterned photoresist layer.

As shown in FIG. 1J, the dummy gate stack 160 in the region 100B isremoved, in accordance with some embodiments. As a result, a recess 230is formed over the semiconductor substrate 100 in the region 100B. Insome embodiments, the portions 110A of the semiconductor wires 110′ andthe portions 120A of the semiconductor wires 120′ are exposed from therecess 230, as shown in FIG. 1J. The portions 110B of the semiconductorwires 110′ and the portions 120B of the semiconductor wires 120′ remaincovered by the spacer elements 200, as shown in FIG. 1J.

As shown in FIG. 1K, the portions 120A of the semiconductor wires 120′are removed, in accordance with some embodiments. As a result, theportions 110A of the semiconductor wires 110′ suspend between the spacerelements 200. The portions 110A serve as channel regions of FETs in theregion 100B.

In some embodiments, the portions 110A have a rectangularcross-sectional profile. The corners and/or edges of the portions 110Amay be relatively rounded. In some other embodiments, the portions 110Ahave an oval cross-sectional profile. In some embodiments, thecross-sectional profile of the portions 110A is not square or circular.

The portions 120B of the semiconductor wires 120′ remain covered by thespacer elements 200, as shown in FIG. 1K. The portions 110A areseparated from one another by an interval that is substantially equal tothe thickness T₂ of the portions 120B. The interval between the portions110A is different from the thickness T₁ of the portions 110A. In someembodiments, the interval between the portions 110A is less than thethickness T₁. The interval between the portions 110A is different fromthe interval between the portions 120A.

In some embodiments, an etching process is performed to selectivelyremove the portions 120A of the semiconductor wires 120′ withoutremoving the portions 110A of the semiconductor wires 110′. In someembodiments, the etchant used in the etching process includes NH₄OH,APM, TMAH, CF₄, SF₆, CH₃F, another suitable solution or gas, or acombination thereof. During the etching operations, the composition ofthe etchant may be varied according to requirements. In someembodiments, the etchant has a sufficiently high etch selectivity of thesemiconductor wires 120′ to the semiconductor wires 110′. As a result,the semiconductor wires 120′ are etched faster than the semiconductorwires 110′.

The etchant used during the stages shown in FIG. 1K is different fromthat used during the stages shown in FIG. 1H. In some embodiments, anetchant for removing the portions 110A and another etchant for removingthe portions 120A have different etch selectivity. For example, theetchant for removing the portions 120A has a lower etch selectivity thanthe etchant for removing the portions 110A. As a result, as shown inFIG. 1K, the portions 110A are partially removed and become smallerduring the removal of the portions 120A due to the loading effect of theselective etching processes. On the other hand, the portions 120A remainsubstantially the same size during the removal of the portions 110A, asshown in FIG. 1H.

As shown in FIG. 1L, a metal gate stack 240 is formed in the recess 230in the region 100B, in accordance with some embodiments. The metal gatestack 240 encircles the portions 110A of the semiconductor wires 110′.For illustration purposes, the portions 110A covered by the metal gatestack 240 are not shown in FIG. 1L. In some embodiments, since theportions 110A are partially removed after the removal of the portions120A, the metal gate stack 240 is in direct contact with both theportions 110A and the portions 110B, as shown in FIGS. 1K and 1L.

As shown in FIG. 1L, the metal gate stack 240 includes an interfaciallayer 250, a gate dielectric layer 260, and metal gate stacking layers270, 280 and 290, in accordance with some embodiments. The materialsand/or formation methods of the metal gate stack 240 in the region 100Bare the same as or similar to those of the metal gate stack 240 in theregion 100A, and are therefore not repeated. The metal gate stack 240 inthe region 100B may include different work function layers than themetal gate stack 240 in the region 100A to adjust the work functionlevel. For example, the work function layer in the region 100A may be anN-type metal layer so as to form an N-type transistor while the workfunction layer in the region 100B may be a P-type metal layer so as toform a P-type transistor.

FIG. 1L-1 is a cross-sectional view of the semiconductor devicestructure taken along a sectional line I-I′ shown in FIG. 1L, inaccordance with some embodiments. FIG. 1L-2 is a cross-sectional view ofthe semiconductor device structure taken along a sectional line II-II′shown in FIG. 1L, in accordance with some embodiments. FIG. 1L-3 is across-sectional view of the semiconductor device structure taken along asectional line III-III′ shown in FIG. 1L, in accordance with someembodiments.

As shown in FIG. 1L-1, the portions 110A of the semiconductor wires 110′are surrounded by the metal gate stack 240 in the region 100B, inaccordance with some embodiments. In some embodiments, the portions 110Aof the semiconductor wires 110′ and the portions 120A of thesemiconductor wires 120′ have substantially the same length butdifferent thicknesses, as shown in FIGS. 1I-1 and 1L-1. The portions110B of the semiconductor wires 110′ are vertically sandwiched betweenthe portions 120B of the semiconductor wires 120′. The portions 120C ofthe semiconductor wires 120′ are surrounded by the source or drainstructures 210.

As shown in FIG. 1L-2, the portions 110A of the semiconductor wires 110′are embedded in the metal gate stack 240 in the region 100B, inaccordance with some embodiments. In some embodiments, the interfaciallayer 250 and the gate dielectric layer 260 are between two of theportions 110A. None of the metal gate stacking layers 270, 280 and 290is between two of the portions 110A.

The space between the portions 110A is not large, so only a part of themetal gate stack 240 is formed between the portions 110A, as shown inFIGS. 1L-1 and 1L-2. There is substantially no work function layerbetween the portions 110A. The metal gate stacking layers 270, 280 and290 cover side surfaces of the portions 110A without covering both topand bottom surfaces of the portions 110A. The uniform work functionlevel from side surfaces results in uniform threshold voltage.Therefore, the device performance of the semiconductor device structureat on-state is improved and current leakage at off-state is avoided. Thecontrol of threshold voltage is enhanced.

In some embodiments, the portions 110A have a decreased width W₂ that isless than the original width W₁. The width W₂ is along the [100]crystalline direction. In some embodiments, the width W₂ is in a rangefrom about 4 nm to about 6 nm. In some embodiments, the width W₂ is lessthan the thickness T₁ of the portions 110A. In some embodiments, a ratioof the thickness T₁ to the width W₂ is in a range from about 1.2 toabout 1.5. In some embodiments, the width W₂ is substantially equal tothe thickness T₂ of the semiconductor wires 120′. The width W₂ may beless or greater than the thickness T₂. The thickness T₂ is along the[110] crystalline direction.

In some embodiments, the portions 110A shown in FIG. 1L-2 havesubstantially the same perimeter as the portions 120A shown in FIG.1I-2. In some embodiments, the portions 110A shown in FIG. 1L-2 havesubstantially the same area as the portions 120A shown in FIG. 1I-2. Thearea is a cross-sectional area taken along the sectional line II-II′shown in FIGS. 1I and 1L.

In accordance with some embodiments, the thickness T₁ of the portions110A is greater than the width W₂ of the portions 110A, as shown in FIG.1L-2. The mobility of holes in the portions 110A (such as silicongermanium) is large or the maximum in the (110) crystalline plane and inthe [110] crystalline direction, as indicated by the dashed arrow inFIG. 1L-2. The thickness T₁ is along the [110] crystalline direction. Asa result, the effective area of channel regions or W_(eff) of thetransistor in the region 100B is greatly increased. The operation speedof the transistor in the region 100B can be improved. Furthermore, thewidth W₂ of the portions 110A is less than the original width W₁ of theportions 110A. Drain-induced barrier lowing (DIBL) effect and/orreduction of subthreshold swing are prevented. The short channel effectcan be controlled. Accordingly, the device performance of thesemiconductor device structure is significantly enhanced.

The mobility of electrons in the material of the portions 120A may begreater than the mobility of holes in the material of the portions 110A.The semiconductor device structure with a fixed total thickness of thesemiconductor wires 110′ and 120′ sacrifices the area of the portions120A to expand the area of the portions 110A. As a result, the mobilityof holes in the portions 110A is improved. The semiconductor devicestructure has better-balanced performance among its differenttransistors or can meet various performance requirements for thedifferent transistors.

As shown in FIG. 1L-3, the vertically stacked portions 110B and 120B arecovered by the spacer elements 200 in the region 100B, in accordancewith some embodiments. In some embodiments, the structure shown in FIG.1L-3 is substantially the same as that shown in FIG. 1I-3, and aretherefore not repeated. The portions 110B shown in FIG. 1L-3 have alarger area than the portions 110A shown in FIG. 1L-2.

In accordance with some embodiments, the portions 110B surrounded by thespacer elements 200 have a larger area. As a result, parasiticresistance of lightly doped drain (R_(LDD)) in the region 100B isgreatly reduced. On the other hand, dopants in the portions 120B (suchas phosphorus in silicon) may diffuse faster than dopants in theportions 110B (such as boron in silicon germanium). Accordingly, even ifthe portions 120B have a smaller area than the portions 110B, R_(LDD) inthe region 100A is still small. Therefore, the device performance of thesemiconductor device structure is improved by enlarging the portions110B and shrinking the portions 120B. The semiconductor device structureremains at a smaller size.

Many variations and/or modifications can be made to embodiments of thedisclosure. For example, the semiconductor device structure is notlimited to including semiconductor wires embedded in source or drainstructures. FIGS. 2A and 2B are perspective views of various stages of aprocess for forming a semiconductor device structure, in accordance withsome embodiments. As shown in FIGS. 2A and 2B, semiconductor devicestructures similar to those shown in FIGS. 1E, 1I and 1L are provided.In some embodiments, the materials, formation methods, and/or benefitsillustrated in the aforementioned embodiments can also be applied in theembodiments illustrated in FIGS. 2A and 2B, and are therefore notrepeated.

As shown in FIG. 2A, the portions of the semiconductor wires 110′ and120′ that are not under the dummy gate stack 160 and the spacer elements200 are both removed, in accordance with some embodiments. As a result,space is created for the formation of the source or drain structures210, as shown in FIG. 2B. In some embodiments, the source or drainstructures 210 is in direct contact with the semiconductor wires 110′and 120′. In some other embodiments, the source or drain structures 210are separated from the semiconductor wires 110′ and 120′ by a linerlayer (not shown). The liner layer may serve as an etch stop layerduring subsequent etching processes to protect the source or drainstructures 210 and reduce parasitic capacitance.

Embodiments of the disclosure are not limited and may be applied tofabrication processes for any suitable technology generation. Varioustechnology generations include a 2 nm node or other suitable nodesbeyond the 2 nm node, such as a 20 nm node, a 16 nm node or a 10 nmnode.

Embodiments of the disclosure form a semiconductor device structure withGAA transistor structures. The semiconductor device structure iscompatible with the general GAA process. Alternately stacked thicker andthinner semiconductor wires result in better and uniform metal-fillingduring the formation of the metal gate stack. The difficulty to form themetal gate stack in a small space is lowered without enlarging the totalthickness of the semiconductor wires. Uniformity of the work functionlevel and threshold voltage can also be achieved.

According to the aforementioned embodiments, the semiconductor devicestructure has a larger effective area of channel regions in multipleFETs (such as an N-type FET and a P-type FET). The operation speed ofthe FETs is greatly improved. Accordingly, the device performance of thesemiconductor device structure is enhanced even further. The effectivearea of channel regions in N-type and P-type FETs both becomeadjustable. The levels of performance of N-type and P-type FETs can bebetter balanced to meet requirements.

In accordance with some embodiments, a semiconductor device structure isprovided. The semiconductor device structure includes firstsemiconductor wires over a semiconductor substrate. The firstsemiconductor wires are vertically spaced apart from each other. Thesemiconductor device structure also includes a gate stack surroundingfirst portions of the first semiconductor wires. The first portions havea first width. The semiconductor device structure also includes a spacerelement surrounding second portions of the first semiconductor wires.The second portions have a second width. In addition, the semiconductordevice structure includes a second semiconductor wire between the secondportions. The second semiconductor wire has a third width. The thirdwidth is substantially equal to the second width and greater than thefirst width.

In accordance with some embodiments, a semiconductor device structure isprovided. The semiconductor device structure includes firstsemiconductor wires over a semiconductor substrate. The firstsemiconductor wires are vertically spaced apart from each other. Thesemiconductor device structure also includes a gate stack surroundingfirst portions of the first semiconductor wires. The first portions havea first width and a first thickness to define a first cross-sectionalarea. The semiconductor device structure also includes a spacer elementsurrounding second portions of the first semiconductor wires. The secondportions have a second width and a second thickness to define a secondcross-sectional area. In addition, the semiconductor device structurealso includes a second semiconductor wire between the second portions.The second semiconductor wire has a third width and a third thickness todefine a third cross-sectional area. The third width is substantiallyequal to the second width and greater than the first width. The firstcross-sectional area, the second cross-sectional area, and the thirdcross-sectional area are different from each other.

In accordance with some embodiments, a semiconductor device structure isprovided. The semiconductor device structure includes firstsemiconductor wires over a semiconductor substrate and vertically spacedapart from each other. Each of the first semiconductor wires includes afirst portion surrounded by a first gate stack, and a second portionsurrounded by a first spacer element. The semiconductor device structurealso includes a second semiconductor wire between the second portions ofthe first semiconductor wires. The second semiconductor wire has a firstwidth that is greater than a second width of the first portions of thefirst semiconductor wires and substantially equal to a third width ofthe second portions of the first semiconductor wires. The semiconductordevice structure also includes third semiconductor wires over asemiconductor substrate and vertically spaced apart from each other,wherein each of the third semiconductor wires includes a first portionsurrounded by a second gate stack, and a second portion surrounded by asecond spacer element. The semiconductor device structure also includesa fourth semiconductor wire between the second portions of the thirdsemiconductor wires. The fourth semiconductor wire has a fourth widththat is substantially equal to a fifth width of the first portions ofthe third semiconductor wires and a sixth width of the second portionsof the third semiconductor wires.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device structure, comprising:first semiconductor wires over a semiconductor substrate, wherein thefirst semiconductor wires are vertically spaced apart from each other; agate stack surrounding first portions of the first semiconductor wires,wherein the first portions have a first width; a spacer elementsurrounding second portions of the first semiconductor wires, whereinthe second portions have a second width; a second semiconductor wirebetween the second portions, wherein the second semiconductor wire has athird width, and wherein the third width is substantially equal to thesecond width and greater than the first width, and source or drainstructures formed adjacent to the gate stack, wherein a portion of thesecond semiconductor wire is embedded in the source or drain structures.2. The semiconductor device structure as claimed in claim 1, wherein thefirst semiconductor wires and the second semiconductor wire comprisedifferent materials.
 3. The semiconductor device structure as claimed inclaim 1, wherein an interval between the first semiconductor wires isless than a thickness of the first semiconductor wires.
 4. Thesemiconductor device structure as claimed in claim 1, wherein a firstcross-sectional area of the first portions of the first semiconductorwires is less than a second cross-sectional area of the second portionsof the first semiconductor wires.
 5. The semiconductor device structureas claimed in claim 4, wherein a third cross-sectional area of thesecond semiconductor wire is less than the second cross-sectional area.6. The semiconductor device structure as claimed in claim 1, wherein thegate stack is in contact with the first portions and the second portionsof the first semiconductor wires.
 7. The semiconductor device structureas claimed in claim 1, wherein the gate stack comprises an interfaciallayer, a gate dielectric layer, and metal gate stacking layers, andwherein the interfacial layer and the gate dielectric layer fill up aspace between the first semiconductor wires.
 8. The semiconductor devicestructure as claimed in claim 7, wherein the metal gate stacking layersdo not extend into the space between the first semiconductor wires.
 9. Asemiconductor device structure, comprising: first semiconductor wiresover a semiconductor substrate, wherein the first semiconductor wiresare vertically spaced apart from each other; a gate stack surroundingfirst portions of the first semiconductor wires, wherein the firstportions have a first width and a first thickness to define a firstcross-sectional area; a spacer element surrounding second portions ofthe first semiconductor wires, wherein the second portions have a secondwidth and a second thickness to define a second cross-sectional area; asecond semiconductor wire between the second portions, wherein thesecond semiconductor wire has a third width and a third thickness todefine a third cross-sectional area, wherein the third width issubstantially equal to the second width and greater than the firstwidth, and wherein the first cross-sectional area, the secondcross-sectional area, and the third cross-sectional area are differentfrom each other; and a dielectric layer formed adjacent to the gatestack, wherein a portion of the second semiconductor wire is directlybelow a dielectric layer.
 10. The semiconductor device structure asclaimed in claim 9, wherein the first semiconductor wires and the secondsemiconductor wire comprise different materials.
 11. The semiconductordevice structure as claimed in claim 9, wherein an interval between thefirst semiconductor wires is less than the first thickness.
 12. Thesemiconductor device structure as claimed in claim 9, wherein the thirdthickness is less than the first thickness.
 13. The semiconductor devicestructure as claimed in claim 9, wherein second thickness issubstantially equal to the first thickness.
 14. The semiconductor devicestructure as claimed in claim 9, wherein the first cross-sectional areaand the third cross-sectional area are less than the secondcross-sectional area.
 15. The semiconductor device structure as claimedin claim 9, wherein the gate stack is in contact with the first portionsand the second portions of the first semiconductor wires.
 16. Asemiconductor device structure, comprising: first semiconductor wiresover a semiconductor substrate and vertically spaced apart from eachother, wherein each of the first semiconductor wires comprises a firstportion surrounded by a first gate stack, and a second portionsurrounded by a first spacer element; a second semiconductor wirebetween the second portions of the first semiconductor wires, whereinthe second semiconductor wire has a first width that is greater than asecond width of the first portions of the first semiconductor wires andsubstantially equal to a third width of the second portions of the firstsemiconductor wires; third semiconductor wires over the semiconductorsubstrate and vertically spaced apart from each other, wherein each ofthe third semiconductor wires comprises a first portion surrounded by asecond gate stack, a second portion surrounded by a second spacerelement, and a third portion surrounded by source or drain structures;and a fourth semiconductor wire between the second portions of the thirdsemiconductor wires, wherein the fourth semiconductor wire has a fourthwidth that is substantially equal to a fifth width of the first portionsof the third semiconductor wires and a sixth width of the secondportions of the third semiconductor wires.
 17. The semiconductor devicestructure as claimed in claim 16, wherein the first semiconductor wiresand the second semiconductor wire comprise different materials, thefirst semiconductor wires and the fourth semiconductor wire comprise thesame material, and the third semiconductor wires and the secondsemiconductor wire comprise the same material.
 18. The semiconductordevice structure as claimed in claim 16, wherein an interval between thefirst semiconductor wires is less than a thickness of the firstsemiconductor wires, and wherein an interval between the thirdsemiconductor wires is greater than a thickness of the thirdsemiconductor wires.
 19. The semiconductor device structure as claimedin claim 16, wherein a first cross-sectional area of the first portionsof the first semiconductor wires is less than a second cross-sectionalarea of the second portions of the first semiconductor wires, and athird cross-sectional area of the second semiconductor wire is less thanthe second cross-sectional area.
 20. The semiconductor device structureas claimed in claim 16, wherein a first cross-sectional area of thefirst portions of the third semiconductor wires is substantially equalto a second cross-sectional area of the second portions of the firstsemiconductor wires, and a third cross-sectional area of the fourthsemiconductor wire is greater than the first cross-sectional area.